1. Field of the Invention
The present invention generally relates to an encapsulated micro electro-mechanical system (MEMS) and methods of making the encapsulated MEMS. In particular, the present invention generally relates to a MEMS structure having an encapsulating layer of a material with different electrical, mechanical and/or magnetic properties from those of a core material, where the encapsulated MEMS structure may be made by complementary metal oxide semiconductor (CMOS) compatible methods.
2. Description of the Related Art
One example of a commonly used MEMS structure is a radio frequency (RF) switch, such RF switches are used in various microwave and millimeter wave applications, such as, tunable preselectors and frequency synthesizers. Semiconductor RF switches are relatively large and bulky, for example, 400 in3 for a 16xc3x9716 array, making packaging sizes for such an array relatively large. Micro-machined RF switches significantly reduce package sizes for such RF switch arrays, for example, down to approximately 1 in3.
FIG. 1 illustrates a commonly used RF switch 100, formed as a monolithically integrated MEMS switch, including a substrate 102, a support 104, and a flexible cantilever beam 106 that is attached at one end to the support 104. The cantilever beam 106 has an electrical contact 112 at its unsupported end, which contacts an underlying contact 114 on the surface of the substrate 102. Electrical contact 114 is usually connected to an RF input signal and forms an RF input port of the RF switch, while the electrical contact 112 forms an RF output port.
The RF switch 100 is actuated by electrostatic forces between a field plate 122 formed on the upper surface of the cantilever beam 106 and a grounding plate 124 located on the surface of the substrate 102. The field plate is connected to a direct current (DC) voltage source, while the grounding plate 124 is connected to ground. As illustrated in FIG. 1, when no voltage is applied to the field plate 122, the electrical contact 112 is separated from electrical contact 114, defining an open contact or OFF state. However, when an appropriate DC voltage is applied to the field plate 122, the flexible cantilever beam 106 is deflected by electrostatic forces, causing the electrical contact 112 to contact electrical contact 114, defining a closed contact or ON state. The closed contact or ON state allows the RF input signal to be electrically connected to the RF output port. When the applied voltage is removed from the field plate 122, the flexible cantilever arm 106 returns to its open contact or OFF state, due to elastic forces inherent to the material of the cantilever beam 106.
However, such cantilever beams are subject to mechanical fatigue and stress, when switched on and off a large number of times. Sometimes, due to prolonged mechanical stress, the cantilever beam will deform and may then be subject to stiction at the electrical contact.
In addition, conventional RF switches that use silicon dioxide, polysilicon or even a composite silicon metal alloy as the beam material, are subject to relatively high insertion losses, which result in reduced sensitivity of the RF switch.
Furthermore, conventional MEMS RF switches frequently use polysilicon beams with electroless plated gold or copper. However, the use of electroless gold plating poses problems during conventional CMOS fabrication processes because there is usually no provision for depositing polysilicon or other similar materials, once a back end of line process, such as, the plating of copper, is started. With no provision for a subsequent front end of line process, electroless plating produces a very rough copper structure that is not passivated to prevent oxidation, electro-migration, and diffusion.
In view of the foregoing and other problems and disadvantages of conventional methods, an advantage of the present invention is to provide an encapsulated MEMS forming an RF switch that may be made of multiple materials, having complementary electrical and mechanical properties, to, for example, reduce metal fatigue and stress during prolonged operation, prevent stiction, and reduce insertion losses.
Another advantage of the present invention is to provide encapsulated MEMS structures that provide various switch structures, such as, encapsulated cantilever beams, encapsulated cantilever beams with one or more electrically-isolated lengths, encapsulated beams fixed at both ends, and encapsulated beams fixed at both ends with one or more electrically-isolated encapsulated lengths. In addition, the encapsulated MEMS may accommodate various numbers of switch contacts and grounding plates and a variety of switch contact and grounding plate configurations on a dielectric layer underlying, for example, an encapsulated beam.
A further advantage of the present invention is to provide an encapsulated MEMS structure that forms an inductive coil, where either the core material or the encapsulating material may comprise a ferromagnetic material in order to enhance inductive performance.
Another further advantage of the present invention is to provide a method of manufacturing the encapsulated MEMS structure that is compatible with CMOS compatible methods.
An additional advantage of the present invention is to provide a method of manufacturing an encapsulated MEMS structure that allows the use of various barrier metals, such as, gold, platinum, palladium, iridium, tungsten, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride and nickel, for encapsulating the MEMS structure, which allows the passivation of an encapsulated inner copper layer and prevents oxidation, electro-migration and diffusion of the copper during subsequent processing.
In order to attain the above and other advantages, according to an exemplary embodiment of the present invention, disclosed herein is a method of fabricating an encapsulated MEMS that includes forming a dielectric layer on a semiconductor substrate, patterning an upper surface of the dielectric layer to form a first trench, forming a release material within the first trench, patterning an upper surface of the release material to form a second trench, forming a first encapsulating layer including sidewalls within the second trench, forming a core layer within the first encapsulating layer, and forming a second encapsulating layer above the core layer in which the second encapsulating layer is connected to the sidewalls of the first encapsulating layer.
According to another exemplary embodiment of the present invention, the first encapsulating layer and the second encapsulating layer are made of barrier metals selected from the group of gold, platinum, palladium, iridium, tungsten, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride, and nickel, while the core layer is made of a semiconductor dielectric material.
According to another exemplary embodiment of the present invention, the method of fabricating an encapsulated MEMS further includes forming a metal layer between the first encapsulating layer and the core layer.
According to another exemplary embodiment of the present invention, the metal layer includes sidewalls that are connected to the second encapsulating layer.
According to another exemplary embodiment of the present invention, forming the metal layer includes depositing an initial metal layer including sidewalls on the first encapsulating layer, depositing a stop layer on exposed surfaces of at least the first encapsulating layer and the sidewalls of the initial metal layer, removing the stop layer located above the sidewalls of the initial metal layer, and recessing the sidewalls of the initial metal layer and of that portion of the stop layer, which adheres to the sidewalls of the initial metal layer.
According to another exemplary embodiment of the present invention, the metal layer comprises a highly conductive metal from the group of copper, gold and aluminum.
According to another exemplary embodiment of the present invention, forming the core layer includes depositing a stop layer on exposed surfaces of at least the first encapsulating layer and the metal layer, depositing a semiconductor dielectric material on the stop layer, planarizing the semiconductor dielectric material to the level of the stop layer, and recessing the semiconductor dielectric material to a level beneath that of the upper surface of the release material to form the core layer.
According to another exemplary embodiment of the present invention, the method of fabricating an encapsulated MEMS further includes forming at least one switch contact and at least one grounding plate on that portion of the dielectric layer that forms a bottom surface of the first trench.
According to another exemplary embodiment of the present invention, removing the release material results in forming a cantilever beam, including at least the first encapsulating layer, the core layer, and the second encapsulating layer, over a bottom surface of the first trench.
According to another exemplary embodiment of the present invention, removing the release material results in forming a beam, supported at both ends, over a bottom surface of the first trench.
According to another exemplary embodiment of the present invention, the first trench and the second trench form coil patterns having two ends, in which an encapsulated inductive coil is formed and removing the release material results in the encapsulated inductive coil being positioned over a lower portion of the dielectric layer and being supported at both ends by higher portions of the dielectric layer.
According to another exemplary embodiment of the present invention, the first encapsulating layer and the second encapsulating layer comprise a ferromagnetic material and the core layer comprises a semiconductor dielectric material or a metal.
According to another exemplary embodiment of the present invention, the first encapsulating layer and the second encapsulating layer include a semiconductor dielectric material or a metal and the core layer includes a ferromagnetic material.
According to another exemplary embodiment of the present invention, a method of fabricating a multilayered metal encapsulated structure for a MEMS, includes forming a base dielectric layer on a semiconductor substrate, patterning an upper surface of the base dielectric layer to form a trench, forming a release material within the trench, forming a first dielectric layer within the release material, patterning the first dielectric layer to form at least two separate trenches along the long axis of the first trench, forming a first metal layer in the at least two separate trenches, forming a second dielectric layer on at least the first metal layer, patterning the second dielectric layer to form two side trenches that contact the first metal layer while retaining a central portion of the second dielectric layer between the two side trenches, forming a second metal layer within the two side trenches, removing areas of the second dielectric layer that surround portions of the multilayered metal encapsulated structure, filling the areas with the release material, forming a third dielectric layer on at least the second metal layer and the central portion of the second dielectric layer, patterning another trench in the third dielectric layer that corresponds to a pattern of the first metal layer and contacts the second metal layer, forming a third metal layer within the another trench of the third dielectric layer, patterning the third dielectric layer to provide access to the release material, and removing the release material to provide the multilayered metal encapsulated structure, a portion of which is separate from and overlies the first dielectric layer.
According to another exemplary embodiment of the present invention, each of the first metal layer, the second metal layer and the third metal layer includes a barrier metal from the group of gold, platinum, palladium, iridium, tungsten, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride and nickel.
According to another exemplary embodiment of the present invention, the method of fabricating the multilayered metal encapsulated structure for a MEMS further includes forming at least one switch contact and at least one grounding plate on that portion of the first dielectric layer that forms a bottom surface of the trench.
According to another exemplary embodiment of the present invention, removing the release material results in forming a cantilever beam over a bottom surface of the first trench.
According to another exemplary embodiment of the present invention, removing the release material results in forming a beam, supported at both ends, over a bottom surface of the first trench.
According to another exemplary embodiment of the present invention, the first trench forms a coil pattern having two ends, and each of the first metal layer, the second metal layer and the third metal layer includes a ferromagnetic material.
According to another exemplary embodiment of the present invention, removing the release material results in forming an inductive coil over a lower portion of the dielectric layer, in which the inductive coil is supported at both ends.
According to another exemplary embodiment of the present invention, a method of fabricating a multilayered metal encapsulated structure, including an electrically-isolated metal encapsulation, for a MEMS includes forming a first dielectric layer on a semiconductor substrate, patterning an upper surface of the first dielectric layer to form a first trench, forming a release material within the first trench, patterning an upper surface of the release material to form at least two separate second trenches along the long axis of the first trench, forming a first metal layer within the at least two separate second trenches, forming a second dielectric layer on at least the first metal layer, patterning the second dielectric layer over each of the at least two separate second trenches to form two side trenches that contact the first metal layer while retaining a central portion of the second dielectric layer between the two side trenches, forming a second metal layer within the two side trenches for each of the at least two separate trenches, removing areas of the second dielectric layer that surround portions of the multilayered metal encapsulated structure, filling the areas with the release material, forming a third dielectric layer on at least the second metal layer and the central portion of the second dielectric layer, patterning a third trench in the third dielectric layer that corresponds to a pattern of the first metal layer and contacts the second metal layer, forming a third metal layer within the third trench, patterning the third dielectric layer to provide access to the release material, and removing the release material to provide the multilayered metal encapsulated structure, a portion of which is separate from and overlies the first dielectric layer.
According to another exemplary embodiment of the present invention, each of the first metal layer, the second metal layer and the third metal layer comprises a barrier metal from the group of gold, platinum, palladium, iridium, tungsten, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride and nickel.
According to another exemplary embodiment of the present invention, the method of fabricating the multilayered metal encapsulated structure, including an electrically-isolated metal encapsulation, for a MEMS further includes forming at least one switch contact and at least one grounding plate on those portions of the dielectric layer that form bottom surfaces of the at least two separate second trenches.
According to another exemplary embodiment of the present invention, removing the release material results in forming a cantilever beam over at least bottom surfaces of the first trench.
According to another exemplary embodiment of the present invention, removing the release material results in forming a beam that is supported at both ends over a bottom surface of the first trench.
According to another exemplary embodiment of the present invention, an encapsulated MEMS includes a dielectric layer formed on a substrate that includes a portion of lesser thickness and at least one portion of a greater thickness, an encapsulated beam where at least one of both ends of the encapsulated beam is supported above the portion of the dielectric layer having a lesser thickness by the corresponding at least one portion of the dielectric layer having a greater thickness, a encapsulating layer that encapsulates the encapsulated beam, and a core layer formed within the encapsulating layer.
According to another exemplary embodiment of the present invention, the encapsulated MEMS further includes a metal layer formed between the encapsulating layer and the core layer, and a stop layer formed between the metal layer and the core layer.
According to another exemplary embodiment of the present invention, the metal layer and the stop layer include sidewalls that are connected to the encapsulating layer.
According to another exemplary embodiment of the present invention, the encapsulated MEMS further includes at least one switch contact and at least one grounding plate on the portion of the dielectric layer having a lesser thickness.
According to another exemplary embodiment of the present invention, the encapsulated beam comprises an encapsulated portion, which includes the encapsulating layer and the core layer, and an electrically-isolating portion, which includes the core layer and is devoid of the encapsulating layer.
According to another exemplary embodiment of the present invention, the encapsulated beam forms a coil pattern, and either the core layer or the encapsulating layer includes a ferromagnetic material.